As is known, an attenuator circuit reduces the power of a signal without distorting or changing other signal parameters (e.g., frequency, waveform shape, etc). As such, attenuators are commonly used for amplitude or gain control in high-frequency applications. Typical attenuator styles include fixed-value attenuators (sometimes referred to as pads), continuously variable attenuators, and digitally programmable attenuators (sometimes referred to as digital attenuators).
Unlike fixed-value and continuously variable attenuators, digital attenuators respond to a control signal by switching in discrete, finite attenuation steps (hence, digital attenuator)'. The control signal is generally provided by the control logic and driver circuitry, which in turn controls the attenuator structure to provide the desired level of attenuation. The control logic and driver circuitry can vary, for example, depending on factors such as the desired complexity, control speed, and specifics of the target application (e.g., room available for attenuator and cost constraints).
Digital attenuators are generally specified by the number of bits of attenuation (e.g., such as a 10-bit attenuator), wherein each bit corresponds to a step of attenuation. The least-significant bit (LSB) selects the smallest single step of attenuation that can be provided by the attenuator and the most-significant bit (MSB) selects the single largest step of attenuation, with the attenuation steps in between these two extremes selected by combinations including the remaining bits. When only the LSB is selected, the attenuator provides its minimum attenuation step, and when all the bits are selected, a digital attenuator provides its maximum attenuation step.
Typical attenuator designs employ a binary weighted switching scheme that operates in conjunction with a network of progressively increasing resistance values (generally referred to as a resistive ladder or ‘R-2R’ network). In particular, when only the LSB is selected, only the smallest resistance value is switched in to set the signal path gain to provide the minimum attenuation step. Likewise, when all the bits are selected, all the resistance values are switched in to set the signal path gain to provide the maximum attenuation step.
There are a number of problems associated with such binary (or other) weighted switching schemes to control signal path gain in conventional attenuator designs, including poor monotonicity and phase discontinuity. There is a need, therefore, for better attenuator designs.